In the treatment of diseased conditions, therapeutic interventions are often undertaken which involve administration of foreign molecules having therapeutically beneficial effects. However, such administrations can often result in unwanted side effects resulting from activation of the body's immune response. Formation of antibodies following administration of therapeutics poses a serious clinical challenge. The antibodies can abrogate activity and/or alter pharmaco-kinetics of the therapeutic molecules.
This is particularly relevant when administering strong antigenic molecules such as peptides, polypeptides or proteins. Many such polypeptides are routinely used as therapeutic molecules. For example, Factor VIII (FVIII) is an essential cofactor in the intrinsic coagulation pathway. Any deficiency or dysfunction of FVIII results in a bleeding disorder, characterized as hemophilia A. Replacement therapy with recombinant FVIII (rFVIII) or plasma-derived FVIII (pdFVIII) is the common therapy for controlling bleeding episodes. FVIII is a multidomain glycoprotein comprising of six domains (A1-A2-B-A3-C1-C2). Prior to secretion into plasma, FVIII is subjected to proteolytic cleavage, leading to the generation of a heterodimer with molecular weights ranging from ˜170 to ˜300 KDa. The presence of the multiple proteolytic sites at the B domain level is responsible for the high heterogeneity of the FVIII preparations. In spite of being FVIII's largest domain (908 amino acids residues or ˜40% of the total number of amino acids residues), the B domain lacks any essential function for the cofactor coagulation activity. Deletion of the B domain leads to a less heterogenic, genetically engineered rFVIII that corresponds to the shortest form of pdFVIII (e.g. ˜170 KDa). B domain deleted rFVIII (BDDrFVIII) is characterized by a higher specific activity than rFVIII and can also be used for treatment of hemophilia.
Another therapeutic molecule is Factor VIIa (FVIIa). This is a trypsin-like serine protease which plays an important role in activating the extrinsic coagulation cascade. FVIIa is a poorly catalytic form of factor VII after the activating cleavage between Arg152 and Ile153. Upon injury, circulating FVIIa becomes an efficient catalyst when forming a complex with tissue factor (TF), its allosteric regulator that is found on the outside of blood vessel. FVIIa-TF complex induces generation of small amounts of thrombin which further triggers blood clotting. Factor VIIa has been approved by the Food and Drug Administration in the United States for uncontrollable bleeding in hemophilia A and B patients who have developed inhibitory antibodies against replacement coagulation factors, factor VIII and factor IX. Intravenous administration of recombinant human Factor FVIIa (rHu-FVIIa) has been introduced because of fewer side effects than other alternative treatment strategies and to circumvent difficulty in preparing plasma-derived FVIIa. However, the short circulation half-life of FVIIa requiring repeated bolus injections to achieve desired efficacy can be problematic.
Additionally, many other proteins are used as therapeutics. These include erythropoietin, VEG-F, other blood coagulation proteins, hormones (such as insulin and growth hormone) and the like. Strategies that can inhibit processing by immune system and also prolong circulation time (reduce frequency of administration) would improve efficacy of proteins. Therefore there is a need in the area of therapeutics to develop formulations that make the proteins less immunogenic, without significantly affecting the circulating time or the efficacy.